Here's the scenario. Say you have a garden variety 8" f/10 SCT. Your use is purely visual, and you are looking to attach to the rear of the OTA whatever optical path add-ons allow you to maximize TFOV while keeping vignetting to a minimum. You have the following items and aren't planning on buying any additional accessories. What is listed is all you have to play with for this exercise.

You have access to pretty much any commercial eyepiece that has been available over the last 20 years, so eyepiece characteristics aren't a factor. I'm interested in what you would choose to place between the rear of the OTA and the eyepiece to accomplish the stated goals.

The standard SCT diagonal is actually one of the shorter configurations.

The reason (I think) that people don't like them though is that they are hard to rotate. To turn them, you have to loosen the collar, and when you tighten them, they sometimes slip loose when using heavy eyepieces tilted to the right side.

There are also shorter options than this.

The Televeue used with the Televue Short Diagonal offers a light path that is a bit shorter than even a standard SCT diagonal. The problem is that you need a diagonal with a short nose, or you need to be willing to cut the nose down on an existing diagonal. I did this using a Televue Everbright. Trimmed about 15mm from the nose so I could use it in a Shorty.

but these are still long.

The shortest possible solution, offering the widest possible true field with no focal reducer is a Baader Diagonal (there is another one that does this too), that has a body threaded with SCT threads.

The diagonal body actually threads directly on to the rear port of a standard SCT.

There is another component called a "Lock Ring" that you screw on first. This ring then is backed against the front of the diagonal so you can rotate the diagonal and lock it into place.

This will be the winner. You can't get shorter than this.

And this. You could also use this to connect a focal reduce directly to the diagonal. If you don't go crazy with the eyepiece, you can get an even wider field, than with a 2" eyepiece alone, though you may loose 10 millimeters or more in aperture, depending on the scope being used.

But assuming you want to run at full aperture with no serious illumination falloff, the Baader Diagonal with SCT threads on the front is as short as you can get for 2" eyepeices. You need the locking wheel though.

The 46mm field stop (actually, it can attain 46.5mm on thinner wall eyepiece barrels) is as big as it gets in 2" eyepieces. The rear opening of the baffle is allowed be rather smaller than this because it is fairly well removed from the focus. The farther a restrictor is from the focus, the more is blurred. And a strongly blurred, out-of-focus restrictor makes for a mess sudden decrease in illumination. In other words, subtler vignetting.

The thread-on SCT 2" diagonals only put the restrictor nearer to the focus, thereby casting a somewhat 'sharper'' shadow and resulting in somewhat more apparent and rapid illumination fall-off toward a large field edge.

As Edggie says, a short optical path length ensures maximal field diameter, due to the effective focal length if the system being kept to a minimum. But I do wonder if the closeness of the focus to the rear opening begins to offset these gains from the standpoint (only!) of edge-of-field illumination...

I hasten to add, even though it lies outside Jim's list, that the Basder solution Edggie advocates, could well be the ultimate solution.

My comments on the possibly increased edge-of-field fall-off (due to reduced distance between rear baffle opening and focus) are, in practical terms, picking nits and splitting hairs. I raised the issue only from the stsndpoint of obtsining a full understanding of the variables, and to allow consideration of the subtler effects if deemed important. Which I feel are not, for the visual observer.

Ok, I remember what happened now. I was using the 8" SCT on an Celestron SE Mount and the Diagonal would hit the base when at Zenith.If I moved the OTA further forward, It threw It out of balance and wouldn't slew properly. So I added a spacer I had received with a WO SCT Focuser and that allowed the necessary clearenace for the Diagonal to clear the Focuser Knob as well a shortening the spacing enough that the OTA was properly balanced and cleared the Mount Base.

Attached Files

The 2" tube, 2" refractor diagonal, and TV 55mm Plossl (or other eyepiece having the maximal 46mm field stop.)

I did this with my first 8" SCT in '85-86, to very good effect. I could not detect illumination fall-off, although I knew it was present.

Glenn, Eddgie, Jim etc:

In purely practical, looking through the eyepiece terms, what does it look like when you stack them all up, F/6.3 focal reducer, 2 inch diagonal, 41mm Panoptic?

I know that the F/6.3 reducer also reduces the effective diameter of the 1.5 inch rear port/rear baffle down to about 24mm, looking though a 24mm rear with a 46mm field stop ought to show some real vignetting. But it's not much different than using a 2 inch diagonal on a C-5 or C-6 with their 1 inch rear port/rear baffle and that actually works reasonably well...

Attached Files

And here's the GSO Diagonal attached to the OTA with the 2" Nosepiece removed and threaded directly to the spacer and then OTA.

Looks like I was mistaken as the Diagonal would still hit the Focuser knob, however I never needed to change the orientation of the Diagonal as the Mount was Alt/Az.
The spacer was in fact needed to allow clearance for the 2" - 1.25" Adapter thumb screws.

Jim, I believe you have a 2" GSO Diagonal already that should work with the addition of a cheap O-Ring.
Mike

Attached Files

I found the off axis loss using a focal reducer and 2" eyepiece to be noticeable.

For example, during the summertime Milky Way, I found that rich star fields would appear more populated in a 2" configuration than in a 2" focal reducer configuration.

But that is me. Many people report not being able to see a difference.

But I could also see the illumination falloff when using even a 22mm Nagler in a MN56 and a 35mm Panoptic in an MN61. It is subtle, but if one studies the field carefully and then changes the configuration, they may see that there are stars in the outer field in the better illuminated scope that are simply missing in the scope with a reducer.

I prefer using a 2" diagonal with a standard low power eyepiece over the focal reducer.

In both cases, even moderately bright stars that are drifted to the field stops will often blink out before hitting the edge.

So for me, Televue Shorty adapter and a short nose diagonal is my preferred configuration. I have tried the focal reducer route many times. I wanted to like it, but felt the view was a tiny bit better with standard 2" 56mm Plossl.

And here's the GSO Diagonal attached to the OTA with the 2" Nosepiece removed and threaded directly to the spacer and then OTA.

Looks like I was mistaken as the Diagonal would still hit the Focuser knob, however I never needed to change the orientation of the Diagonal as the Mount was Alt/Az.
The spacer was in fact needed to allow clearance for the 2" - 1.25" Adapter thumb screws.

Jim, I believe you have a 2" GSO Diagonal already that should work with the addition of a cheap O-Ring.
Mike

Mike, what product are you using for the spacer ring between the scope and the diagonal?

Attached Files

Good question, Jon. I am also curious as to whether a 1.25" prism versus a 1.25" mirror diagonal makes a difference in light path length and illumination. Maybe I'm sensitive to fall off, but the vignetting in a C11 with a 2" SCT thread on diagonal (GSO) and a 34mm Meade Series 5000 SWA was pretty obvious and worse so with the FF/FR in the optical train.

The responses thus far have been interesting, if not, uh, fully "illuminating".

It should be no surprise that you would get a host of very negative behaviors by using a focal reducer in front of a 2" digaonal.

With this configuration, the C11 will start to loose apeture once the back focus total (light path though a focuser, focal reducer, and 2" diagonal) goes past 160mm.

And the apeture loss is severe. With only 180mm of back focs (again, using a focal reducer) the aperture is reduced to 9 incehs.

Worse than that, the illumination will be cut to less than 50% at the edge of an eyepiece with a 30mm field stop (15mm off axis).

Hey, it only matters if it matters though. There are a lot of people that don't seem to mind loosing aperture and field illumintaion.

You may not be one of them though.

Without the focal reducer though, the C11 should handle up to about 300mm of back focus and keep full illumination at the center of the field. The standard C11 has the best field illumination of all the Celestron SCTs that I have data for. There should be a minumum of illumination falloff even with a fairly long ligth path.

So, this is what you may be seeing:

You may just be seeing sky darkening associated with the increase in focal lenght due to your light path lenght and the corresponding increase in focal ratio.

Let's say that your light path is 200mm. With 200mm of back focus, your "f/10" SCT would in fact be working at f/11.5. That is a pretty serious difference.

This, coupled with the slight loss in off axis illumination may indeed cause an observer to see that the field looks blacker at the edge when using the same eyepeices. In fact, the field is blacker at the edge due to a combination of illumination falloff and focal ratio increase (reducing the exit pupil of the observer."

If you would like a copy of Ken Hutchinson's excellent SCT vignetting analysis, just PM me your email address and I will send it along.

I wish there was a way that we could attach it to the forum. It is such an excellent document. Every SCT user should have a copy (Copyright permits it to be shared for this kind of use).

I have become very sensitive to light path lenght in my SCTs and have gone to the shortest possible configurations, even cutting 15mm off of the nose off of a Televue Everbright diagonal so I could use it in a Televue Shorty diagonal.

Everywhere I can, I reduce the back focus. This started when I started using binoviewrs where one has to be extremly careful of back focus.

But I felt the advantages were enough to convert all of my SCTs to as short a light path as possible.

Ed you can upload the file to Dropbox Public folder if you have an account with then and copy and share the public link with us here. The membership is free and it's a great way to share large files.

If, for any reason, you don't want to do this, I will PM you and kindly ask you to send me a copy. Now that i own a C8 Edge, this information is critical. C11 was a bit more generous on this department.